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"chunk": "# Acrylic coatings with surprising antifogging and frost-resisting properties† \n\nCite this: Chem. Commun., 2013, 49, 11764 \n\nJie Zhao,a Anthony Meyer,a Li $\\mathsf{M a}^{\\mathrm{b}}$ and Weihua Ming\\*a \n\nReceived 28th August 2013, Accepted 24th October 2013 \n\nDOI: 10.1039/c3cc46561f www.rsc.org/chemcomm \n\nWe report an unusually effective antifogging/frost-resisting coating based on conventional acrylic polymers. The intriguing antifogging property originated from the delicate balance between the hydrophilicity and hydrophobicity of the acrylic copolymers of 2-(dimethylamino)- ethyl methacrylate and methyl methacrylate, as well as between the water-swellability of the copolymer and the cross-linked network due to ethylene glycol dimethacrylate. \n\nMany antifogging coatings have recently been developed to mitigate fogging problems following changes in environmental conditions for a variety of applications such as eyeglasses, goggles, lenses, mirrors, and display devices in analytical and medical instruments.1–11 Most of the antifogging coatings are hydrophilic or superhydrophilic coatings, primarily due to their ability to significantly reduce light scattering by only allowing water to condensate in a thin-film-like form. Superhydrophilic coatings with water contact angles smaller than $5^{\\circ}$ demonstrate a good antifogging property, but generally require complicated procedures to fabricate surface texture,7,12–15 which is the prerequisite to obtain superhydrophilicity (except superhydrophilic $\\mathrm{TiO}_{2}$ coatings,5,6 which however require UV illumination). In addition, many coatings of this type may not resist frost formation. A superhydrophobic surface with special mosquito-eyelike topography was hypothesized to be antifogging,16 but its fabrication remains a technical challenge. \n\nRecently, it has been demonstrated that antifogging behavior can be obtained by cleverly combining hydrophilic and hydrophobic segments in a coating, such as coatings with both perfluoroalkyl groups and poly(ethylene glycol) (PEG) segments,17 and coatings with zwitter-wettability via layer-by-layer assembly involving PEG segments.18 \n\nWhen a subject surface is in contact with moist air under different conditions (e.g., higher or lower temperature), micrometerscale water (fogging) or ice droplets (frost) may form during the first few seconds of contact. Antifogging behavior at this initial stage is extremely important, since subsequent fogging or frosting may be much less severe or even diminish since the subject has ‘‘adapted’’ to the environmental temperature and humidity after a while. With this understanding, we designed and prepared antifogging acrylic coatings, on the basis of the delicate hydrophilic–hydrophobic balance of the acrylic copolymers of methyl methacrylate (MMA) and 2-(dimethylamino)ethyl methacrylate (DMAEMA), poly(MMA-coDMAEMA), as well as the balance between the water-swellability of the copolymer and the cross-linked network due to ethylene glycol dimethacrylate (EGDMA). \n\nIn our coatings, linear poly(MMA-co-DMAEMA) and cross-linked PEGDMA formed a semi-interpenetrated polymer network (SIPN).19 The molar ratio between the MMA and DMAEMA units was varied to tailor the hydrophilic–hydrophobic balance of the copolymer, which would enable water to diffuse through the coating, yet the polymer did not dissolve in water. In the meantime, the presence of the PEGDMA network would prevent the copolymer from being overswollen by water, thus ensuring coating stability. \n\nWe first synthesized poly(MMA-co-DMAEMA)s by free radical polymerization (details in $\\mathrm{ESI\\dagger}$ ) with the following MMA/DMAEMA molar ratios: $50/50,40/60,30/70$ , and $20/80$ The random copolymer (as indicated by a single $T_{\\mathrm{g}},\\mathrm{ESI}\\dag$ ) was then dissolved in toluene $(10~\\mathrm{wt\\%})$ together with different amounts of EGDMA $(0.1\\mathrm{-}2\\mathrm{wt\\%}$ relative to the copolymer), spin-coated on glass slides, and finally cured by UV $(\\mathrm{ESI\\dag})$ . The final smooth coatings ( $\\sim450\\ \\mathrm{nm}$ in thickness), containing $0.5\\mathrm{wt\\%}$ polymerized EGDMA unless otherwise stated, were labeled according to the DMAEMA content; for instance, Copolymer-70 indicates that the DMAEMA molar content was $70\\%$ in the copolymer. \n\nVarious samples were first stored in a freezer at $-20~^{\\circ}\\mathbf{C}$ for $30~\\mathrm{min}$ , and photographs were taken after the sample was exposed to ambient conditions for 5 s. The control, a hydrophilic glass, fogged severely (Fig. 1a), so did a hydrophobically (perfluoroalkyl, Rf)-modified glass (Fig. 1b). Apparently, typical hydrophobic coatings are not suitable for antifogging applications.18 The observed fog was initially frost, which turned into fog as the sample temperature increased. In sharp contrast, there was neither frost nor fog formation at all (Fig. 1c) for Copolymer-70; excellent clarity was obviously maintained. The sample Copolymer-60 showed some improvement but the antifogging performance was not as good as Copolymer-70. These results clearly suggested that the hydrophilic DMAEMA units played a crucial role in antifogging performance. \n\n \nFig. 1 Photos of different glass slides: (a) control glass, (b) Rf-modified glass, (c) Copolymer-70, and (d) Copolymer-60, first stored at $-20^{\\circ}\\mathsf C$ for $30~\\mathrm{{min}}$ and then exposed to ambient lab conditions for 5 s. \n\nTo evaluate the antifogging performance more quantitatively, light transmission over the $400{\\mathrm{-}}700\\ \\mathrm{nm}$ range was collected on an Agilent 8453 UV-vis spectrophotometer. Prior to fogging tests, Copolymer-70 and Copolymer-60 coatings on glass exhibited light transmission as high as the control glass $(\\sim92\\%$ , Fig. 2a) over the wavelength range of $400{\\mathrm{-}}700\\ \\mathrm{nm}$ , indicating that the random copolymer layer had a negligible effect on glass transmittance. However, the transmission on the Rf-modified sample was significantly lower $(\\sim77\\%)$ . \n\nAfter being subjected to the same frosting/fogging test as above, the light transmission was again monitored. In the case of the control glass and Rf-modified glass, the light transmission decreased to below $20\\%$ (Fig. 2b), obviously due to severe fogging/frosting. On the copolymer coating surface, there appeared to be a strong dependence of transmission on the DMAEMA content (Fig. 2b). Both Copolymer70 and Copolymer-80 maintained high transmission $(>90\\%).$ on par with the values before the fogging test, which again confirmed that fog/frost formation on these surfaces was completely suppressed. In contrast, a significant decrease in transmission was observed on Copolymer-60 and Copolymer-50, obviously due to the lower DMAEMA content in these coatings. In the copolymer coatings, the DMAEMA content should be high enough to attract water molecules to diffuse into the polymer layer; however, excessive DMAEMA units (in the case of Copolymer-80) would lead to over-swelling of the polymer by water (despite the PEGDMA network), thus reducing the stability of the coating upon contact with water. Therefore, there is a critical balance between water-swellability and coating stability, and Copolymer-70 with $70\\%$ DMAEMA in the copolymer $(T_{\\mathrm{{g}}}{:40}\\ ^{\\circ}{\\bf{C}};{\\bf{E S I}}{\\dag})$ , which was coupled with $0.5~\\mathrm{\\wt\\%}$ cross-linked PEGDMA (shown below), was the optimal coating with excellent antifogging/frostresisting performance without compromising coating stability. \n\nTo optimize light transmission and coating stability, we varied the EGDMA content $(0.1{-}2~\\mathrm{wt\\%}$ against the copolymer content) in Copolymer-70 and subjected the samples to a similar fogging test. The light transmission of the samples with high EGDMA contents (1 and $2\\ \\mathrm{wt\\%}$ ) was significantly lower than their counterparts with lower EGDMA contents (Fig. 3). Higher EGDMA contents, after polymerization, led to a dense cross-linked network that would likely restrict the copolymer chain mobility when water molecules diffused into and swelled the copolymer, resulting in lower antifogging capability. With the lower contents of EGDMA (0.1 and $0.5\\mathrm{wt\\%}$ , the cross-link network would be more diluted (compared to the samples with higher EGDMA contents), allowing the copolymer to swell to a greater extent by water and leading to better antifogging performance. However, when the EGDMA content was too low $\\left(0.1\\mathrm{wt\\%}\\right)$ , the coating appeared to be less stable, as indicated by blushing when the coating was submerged in water for $24\\mathrm{~h~}$ . Therefore, there is also an intricate interplay between coating swellability, cross-link network, and coating stability to achieve the best possible antifogging performance for the copolymer-based coatings. Copolymer-70 with $0.5~\\mathrm{wt\\%}$ of EGDMA appeared to be the optimum combination. \n\nTo reveal the origin of the antifogging/frost-resisting properties of the copolymer coatings, we monitored the water contact angle (CA) \n\n \nFig. 2 Light transmission at the normal incident angle for various samples: (a) as-prepared samples and (b) 5 s under ambient conditions after being stored at $-20^{\\circ}C$ for $30~\\mathrm{min}$ . The spikes in the spectra were due to the light bulb. \n\n \nFig. 3 Light transmission at the normal incident angle for Copolymer-70 with different amounts of EGDMA, 5 s under ambient conditions after being stored at $-20^{\\circ}\\mathsf C$ for $30~\\mathrm{min}$ . \n\n \nFig. 4 (a) Water contact angle evolution on various samples as a function of time. (b) Diameter change of the water droplet on various samples over the $600\\varsigma$ period, expressed as $\\Delta D/D_{0},$ where $\\Delta D=D-D_{0},$ and $D_{0}$ and $D$ are the initial diameter (time zero) and the diameter at different times, respectively, of the wetted area by the water droplet. \n\nchange on these surfaces under ambient conditions. During the $600\\mathrm{~s~}$ period, all the water CAs decreased (Fig. 4a), in part due to water evaporation; for instance, CA decreased by $10^{\\circ}$ on the control glass and Rf-Si-modified glass, about $13^{\\circ}$ for Copolymer-50 and Copolymer-60. On the other hand, more than $20^{\\circ}$ of CA decrease was observed on both Copolymer-70 and -80 surfaces, clearly suggesting that some water had gone somewhere else other than getting evaporated. The initial CAs for the copolymers were $60–70^{\\circ}$ , demonstrating that a coating does not have to be superhydrophilic to be effectively antifogging (similar to recent findings by Rubner and Cohen et al.18). We also simultaneously monitored the change in the diameter of the water contact area on the surface (Fig. 4b). No change in the diameter was observed for Rf-Si-modified glass, and there was even slight decrease for the control glass. In contrast, the diameter increased on all four copolymer-based surfaces: ${\\sim}12\\%$ for Copolymer-70 and $18\\%$ for Copolymer-80, respectively, and smaller increase for other two coatings with lower DMAEMA contents over the 600 s period. This observation definitely suggests that water had diffused into the copolymer coating, causing the expansion of the droplet contact area with the polymer surface, and the more DMAEMA segments in the coating, the more significant the water diffusion became. This remarkable water-absorbing capability, coupled with the coating stability due to the cross-linked PEGDMA network, contributed to the excellent antifogging/ frost-resisting properties of Copolymer-70. \n\nThe Copolymer-70 coating was also exposed to boiling water steam; when the time of exposure was less than 5 s, no fogging occurred, but the surface did fog after longer periods of exposure. A possible cause for the poor antifogging behavior at high temperatures is the low critical solution temperature (LCST) of DMAEMA-based polymers. Pure PDMAEMA has a LCST of 38 to $40^{\\circ}\\mathrm{C}$ in water,20 so the copolymer with $70\\%$ DMAEMA was expected to have a slightly higher LCST. When the copolymer was exposed to boiling water steam, the temperature of the copolymer would increase to be above its LCST, making the copolymer no longer hydrophilic. As a consequence, water molecules could not diffuse into the polymer layer, leading to poor antifogging performance. Work is underway to employ polymer systems without this LCST issue, to obtain antifogging coatings at high temperatures. \n\nA possible antifogging mechanism for this new type of antifogging coatings is as follows. When molecular water in moist air from either a warmer or colder environment starts to condensate on the antifogging surface, the water molecules are immediately and rapidly absorbed into the hydrophilic segments of the copolymer (Fig. 5), not allowing (micro)droplets to form on the coating surface (fogging or frosting). Once inside the copolymer coating, water molecules may exist in the nonfreezing state,18 due to the strong polymer–water hydrogen-bonding,21,22 avoiding formation of a large light-scattering water domain. \n\n \nFig. 5 Schematic illustration of the antifogging mechanism in the copolymer coating. \n\nIn conclusion, we have prepared very effective antifogging/frostresisting coatings based on simple acrylic copolymers. The experimental procedure was simple and straightforward. The antifogging/ frost-resisting properties originated from the delicate balance between the hydrophilicity and hydrophobicity of the acrylic copolymer, as well as between the water-swellability of the copolymer and the cross-linked network. Due to its simplicity, this type of coating may be widely applicable in display devices, optical lenses, eyeglasses, mirrors, and other areas. \n\nFinancial support from USDA/NIFA is gratefully acknowledged.",
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"category": " Results and discussion"
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"chunk": "# Notes and references \n\n1 L. Zhang, Y. Li, J. Sun and J. Shen, Langmuir, 2008, 24, 10851. \n2 L. Zhang, Z. Qiao, M. Zheng, Q. Huo and J. Sun, J. Mater. Chem., 2010, 20, 6125. \n3 J. R. Premkumara and S. B. Khoo, Chem. Commun., 2005, 640. \n4 N. Nuraje, R. Asmatulu, R. E. Cohen and M. F. Rubner, Langmuir, 2011, 27, 782. 5 R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima and A. Kitamura, Nature, 1997, 388, 431. \n6 R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi and T. Watanabe, Adv. Mater., 1998, 10, 135. \n7 (a) F. C. Cebeci, Z. Z. Wu, L. Zhai, R. E. Cohen and M. F. Rubner, Langmuir, 2006, 22, 2856; (b) N. Nuraje, R. A. Asmatulu, R. E. Cohen and M. F. Rubner, Langmuir, 2011, 27, 782. \n8 P. Chevallier, S. Turgeon, C. Sarra-Bournet, R. Turcotte and G. Laroche, ACS Appl. Mater. Interfaces, 2011, 3, 750. \n9 X. Liu, X. Du and J. He, ChemPhysChem, 2008, 9, 305. \n10 J. A. Howarter, K. L. Genson and J. P. Youngblood, ACS Appl. Mater. Interfaces, 2011, 3, 2022. \n11 L. Xu and J. He, ACS Appl. Mater. Interfaces, 2012, 4, 3293. \n12 J. Xiong, S. N. Das, J. P. Kar, J.-H. Choi and J.-M. Myoung, J. Mater. Chem., 2010, 20, 10246. \n13 N. J. Shirtcliffe, G. McHale, M. I. Newton, C. C. Perry and P. Roach, Chem. Commun., 2005, 3135. \n14 D. Tahk, T. Kim, H. Yoon, M. Choi, K. Shin and K. Y. Suh, Langmuir, 2010, 26, 2240. \n15 X. Li and J. He, ACS Appl. Mater. Interfaces, 2012, 4, 2204. \n16 X. F Gao, X. Yan, X. Yao, L. Xu, K. Zhang, J. Zhang, B. Yang and L. Jiang, Adv. Mater., 2007, 19, 2213. \n17 J. A. Howarter and J. P. Youngblood, Macromol. Rapid Commun., 2008, 29, 455. \n18 H. Lee, M. L. Alcaraz, M. F. Rubner and R. E. Cohen, ACS Nano, 2013, 7, 2172. \n19 A. Aleman, et al., Pure Appl. Chem., 2007, 79, 1801. \n20 G. Burillo, E. Bucio, E. Arenas and G. P. Lopez, Macromol. Mater. Eng., 2007, 292, 214. \n21 H. Ohno, M. Shibayama and E. Tsuchida, Makromol. Chem., 1983, 184, 1017. \n22 M. S. Sanchez, G. G. Ferrer, M. M. Pradas and J. L. G. Ribelles, Macromolecules, 2003, 36, 860.",
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